The invention involves a device and process for fixing a toner onto a substrate or a printed material, especially a sheet-shaped or a band-shaped printed material, preferably for a digital printer.
In digital printing, especially electrostatic or electrophotographic printing, a latent electrostatic image is generated, which is developed by charged toner particles. These toner particles are transferred onto a printed material, e.g. paper, that receives the image. The image transferred onto the printed material is fixed there by heating and softening of the toner or heating of the printed material. Through and during this process, toner particles bond to the printed material and, possibly, also to each other.
For the fixing of the toner onto the printed material, the use of microwaves is known. Since the absorption of microwave energy in the toner customarily is at least one order of magnitude less than in the printed material, the printed material is preferably heated up by the microwaves and the printed material for its part heats up the toner located on it, and, to be precise, up to a temperature at which the toner bonds to the printed material. As is known, characteristic values of the printed material used, such as, for example, weight, humidity, and composition, are critical in the use of microwaves for fixing of the toner and must be taken into consideration.
Thus, for example, an image-fixing device is known from U.S. Pat. No. 4,511,778, which fixes an image made of toner using high-frequency waves, in particular, microwaves, onto a printed material, especially a sheet of paper. One aspect of the known device is thus the possibility to output the microwaves depending on the size of the printed material, in order to ensure a proper fusing and fixing of the toner taking into account this size as a characteristic value of the printed material. This is a method that only takes into consideration a size of the printed material that is directly apparent and specifies for the operation of the device, prior to fixing, based on consideration, for example, that a larger piece to be heated requires more energy in total than a smaller piece to be heated, because of its larger heat capacity.
However, additional critical aspects remain unconsidered in the use of microwaves for the fixing of toner. Thus, for example, the cited method can only be used in black-white printing with paper weights of a small variation width, while the possibly different behavior of different colored toner and different paper weights, also with possibly different water content, is not considered in this all-inclusive method that is matched to the size of the printed material. In a color print, the toner image can, for example, have four different toner layers. In the process, the maximum density of each toner layer on the image-receiving substrate or printed material is 100%, whereby a maximum total density of the toner layers in the toner image of 400% results. Customarily, the density of a single-color toner image is in the range from 0% to 100% density, and the density of a color toner image is in the range from 0% to 290%. Moreover, the cited device does not contain a microwave resonator, which is desirable when using the microwave application in regard to a homogenous heating, whereby customarily even at least two resonators arranged offset from each other are used, as is known from the patent U.S. Pat. No. 5,536,921 for a general microwave heating outside of the print area.
In addition, during the use of sheet-shaped printed material, a problem can occur that in the area of the edge area of the sheet irradiated with microwaves, processing is done in an energetically different way than the middle sheet area, so that a non-uniformly created printed product can occur. In addition to this, it occurs that during the fixing of traditional toners, only when using microwaves under certain circumstances, only an incomplete melting of the toner is obtained depending on its layer thickness, or heating occurs with bubble formation in areas of the toner. Also, the adhesion of the toner onto the printed material is insufficient under certain circumstances, because, for example, the bond with the printed material is not created sufficiently by the viscosity of the melted toner, which is too high. Problems can occur especially when a printed material is printed on both sides in two subsequently performed printing operations.
Because of these possible problems depicted, the use of microwave radiation in fixing is traditionally and customarily not relied upon, but instead, the toner is in practice heated without microwave radiation and bonded to the printed material using a heated pair of rollers while being impinged with pressure. A non-contact fixing is in principal, however, desirable for the protection of the printed image. Additional advantages of the non-contact fixing are the avoidance of adhesive abrasion and the resultant increased service lifetime of the device used, and an improved reliability of the device.
The purpose of this invention is to make possible an adequate fixing of toner onto a printed material using microwaves, preferably also for a multicolor printing on sheet-shaped printed material and using a resonator and preferably by adjusting to the special prevalent conditions. This purpose is achieved according to the invention in regard to the process in that the printed material that has the toner is irradiated with microwaves from at least one microwave emitter and is heated to melt the toner and that a toner is used which has a sharp transition from its solid to its liquid state during heating.
In this way according to the invention, for example, a dry toner can be used which is still quite hard at an average temperature of approximately 50xc2x0 C. to 70xc2x0 C., so that it can be powdered via conventional processes into a desired average toner size of, for example, 8-4 micrometers and also does not yet become sticky or does not melt at development temperatures, but at a higher temperature of, for example, approximately 90xc2x0 C. is already very fluid at low viscosity, so that it, if necessary in using capillarities, also without outside pressure and in a non-contact manner settles on and in the printed material and adheres and upon a cooling down then becomes hard again very quickly and is fixed. To be precise, the fixed toner has a good surface gloss that is matched to the printed material, especially lacking formed grain boundaries. The surface gloss also plays a direct, meaningful role for color saturation in colored toner.
In this process, in connection with the toner according to the invention, the ratio of the value of the modulus of elasticity Gxe2x80x2 at the reference temperature value, calculated from the starting temperature at the beginning of the glass transformation of the toner plus 50xc2x0 C., to the value of the modulus of elasticity at the starting temperature itself can be  less than 1Exe2x88x925, preferably even  less than 1Exe2x88x927, whereby E represents the base 10 exponent. The starting temperature of the beginning of the glass transformation of the toner is preferably specified as that temperature value at which the tangents to the function progression of the modulus of elasticity Gxe2x80x2, as a function of the temperature before and after the glass transformation, intersect. Preferably, the transformation of the toner from its solid into its liquid state should occur in a temperature interval or temperature window from approximately 30xc2x0 to 50xc2x0 C. in size. This range should be above 60xc2x0 C., preferably approximately between 70xc2x0 C. to 130xc2x0 C., quite preferably between 75xc2x0 C. and 125xc2x0 C.
An additional further embodiment of the process according to the invention is characterized for adjusting to the special conditions in that at least one physical process parameter is controlled or regulated as a function of a parameter that correlates to the energy input into the printed material that has the toner. In this process, the energy input mentioned can essentially correspond to a microwave power that has been absorbed by the entire system out of printed material and toner, so that, according to the invention, corresponding to the actual relationships, the energy that has been output is compared to the absorbed power and tuned. This in turn corresponds essentially to an efficiency control or adjustment. In the process, the performance of a regulation on the emitter in the most general sense or on the absorbing toner-printed material system or on its handling is generally taken into consideration.
For this purpose, the invention preferably proposes in detail to regulate the output of the microwave emitter or to regulate the speed of the movement of the printed material or to tune the resonator or to tune the frequency of the microwaves, and this last measure preferably also in order to achieve a higher energy absorption directly in the toner itself, and in this way to have a more precise influence on its fusing than indirectly and more problematically, via the printed material. As measurable parameters for the dependent regulation, the invention preferably proposes the temperature of the printed material or the microwave energy reflected by the toner-printed material system and thus not absorbed. Additional measurable parameters canxe2x80x94without limitation of themxe2x80x94be the weight/the thickness or the water content of the printed material or density and gloss of the toner layer.
As already mentioned above, in regard to the patent U.S. Pat. No. 5,536,921, at least two resonators for the microwaves, which are offset from each other by xcex/4, in order to offset the maxima of the standing waves in the resonators correspondingly from each other, are customarily required for a homogenous heating of the printed material that is covered with toner. An additional embodiment of the invention instead provides using only one resonator, which oscillates completely or partially.
An additional embodiment of the invention provides during the use of more than two resonators, to offset them by a length of xcex divided by two times the number of resonators. In this way, a more uniform temperature distribution is obtained on the substrate than at an offset of xcex/4. In a preferred embodiment of the invention, four resonators are used whose separation distance each amounts to xcex/8. In principal, all frequencies of the microwave range from 100 MHz to 100 GHz can be used. Usually, the ISM-frequencies released for industrial, scientific or medicinal use, preferably, 2.45 GHz, are used. A use of other frequencies in the wide frequency range mentioned can, however, advantageously lead to a larger portion of the radiation energy being absorbed by the toner than is customary, so that it is not just absorbed by the printed material.
Further, a device of the invention is provided herein for the irradiation and heating of the toner that exhibits a sharp transformation from its solid to its liquid state when heated, there is at least one emitter that outputs microwaves. Preferably one or more operating parameters are additionally provided that can be regulated.
The use of at least one resonator is preferred which has a width of approximately 1 to approximately 10 cm in the movement direction of the printed material, in order to simplify the handling of the printed material. It will also make possible a sufficient power (for example, 1-10 kW per resonator) without voltage break-throughs occurring. In this process, the width of the resonator should also be tuned to the speed of the printed material. This involves a relative speed (for example up to 100 cm/s), in such a manner that the fixing device could move in kinematic reversal relative to the stationary printed material or even both components. Also, a stationary fixing without any movement would be conceivable.
The device according to the invention is preferably provided for a digital multi-color printer.